Piezoelectric materials and magnetostrictive materials (collectively referred to below as xe2x80x9cpiezoelectric materialsxe2x80x9d) deform when an electric field or magnetic field is applied. Thus piezoelectric materials, when used as an actuator, are capable or controlling the relative position of two surfaces.
Piezoelectricity is the general term to describe the property exhibited by certain crystals of becoming electrically polarized when stress is applied to them. Quartz is a good example of a piezoelectric crystal. If stress is applied to such a crystal, it will develop an electric moment proportional to the applied stress.
This is the direct piezoelectric effect. Conversely, if it is placed on an electric field, a piezoelectric crystal changes its shape slightly. This is the inverse piezoelectric effect.
One of the most used piezoelectric materials is the aforementioned quartz. Piezoelectricity is also exhibited by ferroelectric crystals, e.g. tourmaline and Rochelle salt. These already have a spontaneous polarization, and the piezoelectric effect shows up in them as a change in this polarization. Other piezoelectric materials include certain ceramic materials and certain polymer materials. Since they are capable of controlling the relative position of two surfaces, piezoelectric materials have been used in the past as valve actuators and positional controls for microscopes. Piezoelectric materials, especially those of the ceramic type, are capable of generating a large amount of force. However, they are only capable of generating a small displacement when a large voltage is applied. In the case of piezoelectric ceramics, this displacement can be a maximum of 0.1% of the length of the material. Thus, piezoelectric materials have been used as valve actuators and positional controls for applications requiring small displacements.
Two methods of generating more displacement per unit of applied voltage include bimorph assemblies and stack assemblies. Bimorph assemblies have two piezoelectric ceramic materials bonded together and constrained by a rim at their edges, such that when a voltage is applied, one of the piezoelectric material expands. The resulting stress causes the materials to form a dome. The displacement at the center of the dome is larger than the shrinkage or-expansion of the individual materials. However, constraining the rim of the bimorph assembly decreases the amount of available displacement. Moreover, the force generated by a bimorph assembly is significantly lower than the force that is generated by the shrinkage or expansion of the individual materials.
Stack assemblies contain multiple layers of piezoelectric materials interlaced with electrodes that are connected together. A voltage across the electrodes causes the stack to expand or contract. The displacements of the stack are equal to the sum of the displacements of the individual materials. Thus, to achieve reasonable displacement distances, a very high voltage or many layers are required. However, conventional stack actuators lose positional control due to the thermal expansion of the piezoelectric material and the material(s) on which the stack is mounted.
Due to the high strength, or stiffness, of piezoelectric material, it is capable of opening and closing against high forces, such as the force generated by a high pressure acting on a large surface area. Thus, the high strength of the piezoelectric material allows for the use of a large valve opening, which reduces the displacement or actuation necessary to open or close the valve.
With a conventional piezoelectrically actuated relay, the relay is xe2x80x9cclosedxe2x80x9d by moving a mechanical part so that two electrode components come into electrical contact. The relay is xe2x80x9copenedxe2x80x9d by moving the mechanical part so that the electrode components are no longer in electrical contact. The electrical switching point corresponds to the contact between the electrode components of the solid electrodes.
Conventional piezoelectrically actuated relays typically do not possess latching capabilities. Where latching mechanisms do exist in piezoelectrically actuated relays, they make use of residual charges in the piezoelectric material to latch, or they actuate switch contacts that contain a latching mechanism. Prior methods and techniques of latching piezoelectrically actuated relays lacks reliability.
The present invention is directed to a microelectromechanical system (MEMS) actuator assembly. Moreover, the present invention is directed to a piezoelectrically actuated relay that switches and latches.
In accordance with the invention, a piezoelectrically actuated relay that switches and latches by means of a liquid metal is disclosed. The relay operates by means of a longitudinal displacement of a piezoelectric element in extension mode displacing a liquid metal drop and causing it to wet between at least one contact pad on the piezoelectric element or substrate and at least one other fixed pad to close the switch contact. The same motion that causes the liquid metal drop to change position can cause the electrical connection to be broken between the fixed pad and a contact pad on the piezoelectric element or substrate close to it. This motion of the piezoelectric element is rapid and causes the imparted momentum of the liquid metal drop to overcome the surface tension forces that would hold the bulk of the liquid metal drop in contact with the contact pad or pads near the actuating piezoelectric element. The switch latches by means of surface tension and the liquid metal wetting to the contact pads.
The switch can be made using micromachining techniques for small size. Also, the switching time is relatively short because piezoelectrically driven inkjet printheads have firing frequencies of several kHz and the fluid dynamics are much simplified in a switch application. Heat generation is also reduced compared with other MEMS relays that use liquid metal because only the piezoelectric elements and the passage of control and electric currents through the actuators of the switch generate any heat.